As circuits have become more complex, electronic design automation (EDA) software tools have been developed to facilitate designing, testing and modifying circuit designs in preparation for manufacturing (i.e., fabricating or otherwise producing) physical circuit structures that are based on final versions of the circuit designs. As used herein, the term “circuit” generally refer to integrated circuit (IC) devices that are fabricated on semiconductor substrates using CMOS fabrication techniques, and the term “circuit design” refers to a software-based description of a corresponding circuit at a system level, sub-system level, or even at a transistor level. EDA software tools typically include resources (e.g., standard-cell libraries) and design assembly software that allow circuit designers to efficiently arrange and interconnect the various functional sections (blocks) of their circuit designs in the form of software-based circuit descriptions, various testing resources that allow the circuit designers to test and modify their circuit designs during development, and various conversion tools that facilitate the generation of layout files (i.e., software files that include all masks and fabrication process steps, such as diffusion and ion implantation, required to fabricate an associated circuit design using a selected fabrication process, such as CMOS), thereby facilitating subsequent fabrication of physical circuits based on the final circuit designs. Because modern circuits (e.g., System-on-Chip devices) can include billions of transistors and other circuit elements, EDA tools have become essential in the development and testing of modern circuit designs. That is, without EDA software tools, generating a modern circuit from concept to physical circuit using manual design techniques would be practically impossible.
Technology Computer-Aided Design (TCAD) software tools are a type of EDA software tool that allow circuit designers to model (i.e., virtually fabricate) the physical configuration and related device properties of a circuit design, and then simulate operation of the model in response to various environmental conditions before actual fabrication of an physical device (chip) including circuitry that implements the new circuit design. Modelling generally involves utilizing a layout (technology) file and design rules to generate a virtual three-dimensional (3D) physics-based model including all structures and doped semiconductor regions that would be produced physically if the layout file was followed during the selected fabrication process. Simulation and characterization involve virtually testing the physical structures forming the 3D model using a variety of conditions (e.g., various applied voltages and operating temperatures) to determine performance, yield and reliability of the circuit design (i.e., as described by the layout file). TCAD tools are therefore distinguished from other EDA verification tools in that they consider the physical configuration and related device properties of a circuit design.
In addition to simulating and testing electrical (device) characteristics, modern TCAD software tools also facilitate optical simulation of opto-electric elements, such as diodes and image sensors. The optical simulation of an image sensor design using a current commercially available (conventional) TCAD software tool typically begins when a user submits a layout file (aka, recipe card) that includes all process details (e.g., process-emulation commands, mask designs, doping profiles and critical dimension details) needed to fabricate a physical image sensor device based on the image sensor design. Known techniques are utilized to virtually fabricate a 3D model of the image sensor based on the layout file, then modify the 3D model (or a 2D model including a cross-section of the 3D model) to include optical mesh information that describes optical characteristics at each point within the 2D/3D model, and then perform optical simulation to test (characterize) the response of their image circuit designs to applied electro-magnetic radiation at various frequencies. Accordingly, by using a modern TCAD tool, such as Sentaurus TCAD produced by Synopsys, Inc. of Mountain View, Calif., USA, CMOS (and other) image sensor designers are able to virtually construct, simulate and characterize new image sensor designs in a cost-effective manner before actual fabrication of physical test devices in a foundry, thereby substantially reducing both development costs and development time in comparison to foundry-only development techniques.
Although conventional TCAD tools substantially reduce image sensor development cost/time, they require image sensor designers to generate a separate layout file that completely describes each CMOS image sensor (CIS) pixel configuration submitted for simulation. That is, in addition to generating a layout file including a front-end design (i.e., photodiode and control transistors formed in and on a semiconductor substrate), designers must also include in the layout file processing details related to all back-end structures (e.g., metal wiring) and all optically-relevant structures (e.g., color filter layers and micro-lenses). This task is even more time consuming when a designer wishes to compare the characteristics of CIS pixel designs having slightly different configurations because the designer must generate and submit a new layout file for each minor difference. For example, if a designer wished to compare a back-side illuminated (BSI) version of a particular CIS pixel design with a front-side illuminated (FSI) version of the CIS pixel design, the designer would be required to generate and submit a first layout file including a description of the BSI version and a second layout file including a description of the FSI version. Similarly, if a designer wished to slightly change other aspects of a pixel configuration (e.g., to move a micro-lens position, or to add an anti-reflective layer), the designer would have to generate and submit a separate layout file for each different CIS pixel design configuration. As such, is typically very time-consuming for CIS pixel designers to utilize conventional TCAD tools during development of a CIS pixel due to the significant effort required to specify all mask features and process parameters for each different CIS pixel/array configuration, which can lead to development delays, lost revenue, and can limit a designer's creativity and innovation.
In addition to the burden placed on designers, the required submission of separate layout files for each different CIS pixel/array configuration greatly increases the computer processing time required to develop CIS image sensors using conventional TCAD software tools. That is, conventional TCAD software tools perform a complete structural generation (i.e., virtual fabrication) process and a complete mesh generation process for each submitted layout file, even when two sequentially submitted layout files include similar CIS pixel/array configurations. For example, in the above-mentioned case of similar BSI and FSI CIS pixel design versions (i.e., where front-side and metal interconnect structures are essentially identical in each version), conventional TCAD software tools perform substantially identical structural/mesh generation processes twice with respect to the substantially identical structures (i.e., virtual fabrication of front-side and metal interconnect structures is performed a first time when the first layout file including the BSI version is submitted, and then performed a second time when the second layout file including the BSI version is submitted). This practice of processing each submitted layout file “from scratch” produces an inherent inefficiency in that the computer processor executing the TCAD software tool is required to virtually fabricate and generate optical meshes for substantially identical structures. This processing inefficiently is compounded (i.e., made worse) when a designer wishes to compare performance of groups of pixels that utilize different color filter schemes because structure generation must be performed “from scratch” for each group of pixels.
What is needed is an improved TCAD tool and associated method for performing optical simulation and characterization that facilitates efficient image sensor prototyping by way of allowing a designer to avoid the inefficiencies associated with conventional tools/methods during the development of CIS pixel designs.